semi synthetic V mineral oil ...

what are the real world problems on switching between the recommended 5w 30
synth blend oil and 10w 40 mineral oil in a car? I want to use up a load of
10w 40 mineral oil in it. What is likely to go?...Bearing in mind it will
only be used for six months in the summer.

Depends on the engine.
Viscosity-wise, some - especially those that require thin oils like 5w30
- can have all sorts of problems if used with thicker oil.
Base-stock-wise, again depending on the engine, there shouldn't be any
great difference between mineral and semi-synth, except you'll want to
change it more quickly. If it's a turbo, though, I really wouldn't - the
heat of the turbo may very well cause it to carbonise in relatively short
order.
For the price of even five litres of mineral (so cheap) oil, why risk it?
Just chuck it on freecycle or in the lawnmower or even take it down the
tip.

You might be better off asking in a North American group, then, where
they've actually seen that engine.

Can't see the difference being particularly significant to HG life.
TBH, in something of that era and understressedness - especially given
the age of design - I'd probably be quite happy to just do it - but I'd
want to find out if there was a particular reason that viscosity was
recommended. 5w30 is REALLY thin for a '90s lump.

Yes, certainly. 5W is less viscous in cold weather than 10W. The W
stands for Winter BTW. A 5W-30 oil stays more fluid at low
temperatures than a 10W-40 oil, but the latter retains viscosity
better at high temperatures than the former. Good low-temperature
performance is important for starting in ice-cold winter weather if
the car's been outside overnight, and for the first mile or so
thereafter while the engine warms up. Retaining viscosity at high
temperatures would be important if you were planning to drive across
the Sahara in summer, for example.
I can't really see that you'd be troubled by using the 10W-40, unless
Glasgow is hit by an extraordinarily cold winter in 8 months time.
And don't call me Shirley

A distinct possibility, apparently. El Niño, it is said, is all set for
a repeat of the cold snowy winters we experienced five or six years ago.
That means life as we know it comes to a complete standstill south of
Newcastle, but up here in Aberdeenshire life will continue as normal.
Those in Glasgow will just chuck an extra Mars Bar in the deep fat
fryer.

What was the standard change interval...? 3k miles?
Gawd bless the 'merkins. Driving conditions that mean they could easily
and safely extend changes far more than here - yet they change oil almost
every time they switch the damned engine off...

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